The present invention relates to a device for measuring the relative position of two objects comprising a graduation, a plurality of reference marks absolutely allocated to the graduation, and a scanning unit to scan the graduation and reference marks to generate at least one scanning signal and at least one reference signal, respectively.
Relative position measuring instruments are frequently used in processing machines to measure the relative position of a tool with respect to a workpiece. The instruments are also frequently used in coordinate measuring machines for the determination of the dimensions and the relative position of a test object.
In devices of this type, reference signals are generated during the scanning of the reference marks and are utilized in a variety of ways. For example, the reference signals may be used to reproduce the zero position in a counter, for beginning a measurement operation from a particular position, or for the recovery of a measurement position after an interruption in a measurement operation. The reference signals may also be used for communicating to an auxiliary control device connected to the measuring instrument.
German published application DE-AS No. 25 40 412 discloses a measuring instrument in which a measurement embodiment presents a graduation and identical reference marks. The reference marks are spaced with respect to one another and are absolutely allocated to the graduation. At least one selecting element in the form of a magnet slidable in a groove of the scale is provided for the selection of at least one reference mark. A scanning unit scans the graduation and the reference marks to generate at least one periodic scanning signal and at least one reference signal. A switching element contained in the scanning unit is activated when the selection element is scanned by the scanning unit. The switching element generates a selection signal. If the reference signal and the allocated selection signal occur simultaneously, a control signal is generated by a component connected to the outlet side of the measuring instrument.
One disadvantage of this measuring instrument is that, when the instrument is used, for example, on a processing machine, jarrings of the machine can displace the selection element from its desired position along the groove. If the selection element is displaced, the clear allocation of the selection element to the selected reference mark, as well as the activation of the selected reference mark, can no longer be assured. In the case of reference marks positioned closely adjacent one another, the displacements of the selection element can result in the selection of a false reference mark. Such an incorrect selection can result in the initiation of an incorrect control process, for example, the triggering of a zeroing process in a counter thereby resulting in a false measurement result. Additional means ar therefore needed to reproduceably fix each slidable selection element in the groove relative to the particular selected reference mark.
In German Patent DE-PS No. 32 45 914 there is described a measuring in which a measurement embodiment comprises a graduation and a plurality of identical reference marks. The reference marks are spaced with respect to one another and are absolutely allocated to the graduation. A track with at least one electrically applied selection datum is allocated to the reference marks for the selection of at least one reference mark required for the measurement. The selection information is scanned by an element of the scanning unit so that at least one reference mark can be selected and processed.
This type of measuring instrument assures an invariable, clear allocation of the selection information to the selected reference mark. However, the instrument is disadvantageous in that a separate track with the appertaining selection datum is needed for the selection of the reference marks. In addition, an additional element must be provided in the scanning unit for the scanning of the selection information.
In German Patent DE-PS No. 29 52 106 there is disclosed a measuring instrument in which a measurement embodiment comprises a graduation and a plurality of reference marks absolutely allocated to the graduation. Each reference mark presents a different line-group distribution. Within the scanning unit, a respective scanning field is allocated to each reference mark with an identical line-group distribution for the selection of the reference marks. One disadvantage of this arrangement is that it is relatively expensive since the line-group distributions of the individual reference marks and of the scanning fields must differ from one another as much as possible in order to make possible a clear identification of the individual reference marks. Also, the scanning arrangement must have an identical scanning field for each reference mark.
German unexamined patent specification DE-OS No. 30 39 483 discloses a measuring instrument in which a measurement embodiment comprises a graduation and a plurality of identical reference marks. The reference marks are located at equal intervals and are allocated absolutely to the graduation. A code mark is allocated to each reference mark for the identification of the individual identical reference marks. This arrangement has the disadvantage that separate scanning fields on the scanning plate of a scanning unit are required for the scanning of these marks.
German Patent DE-PS No. 24 16 212 discloses a measuring instrument in which a measurement embodiment comprises a graduation and identical reference marks absolutely allocated to the graduation. The absolute value of each reference mark is determined from its particular specific spacing to an adjacent reference mark. The differing spacings between adjacent reference marks are determined by scanning the incremental graduation. The reference mark and the apertaining adjacent reference mark must be scanned and their mutual specific spacing determined for the identification of each individual identical reference mark.
The disadvantage of this measuring instrument is that the greater the measurement length and therewith the number of identical reference marks, the greater the number of different spacings are required. The reference marks with the greatest allocated spacings are positioned so far apart that, for the identification and determination of the absolute values of the reference marks, large graduation distances must be scanned. Thus, the identification of these reference marks with a great number of allocated spacings is time consuming and therefore economically undesirable in a measuring instrument used in a processing machine. If in the processing machine a momentary position measuring value is lost, as for example by a power failure, then, once the disturbance is corrected, the absolute reference position of the measuring instrument must be recovered for further measuring processes. The redetermination of the absolute reference position is performed, for time reasons, by identifying the nearest situated reference mark. To identify the nearest reference mark and determine its absolute value, its apertaining adjacent mark must be scanned and their mutual specific spacing determined by scanning the incremental graduation. If the mutual specific spacing is very great, the identification of the reference mark is time consuming and contributes to the increase in down-time of the processing machine. In addition, the scanning of such great spacings may not be possible due to the particular geometry of the workpiece.
It is an object of the present invention to provide a measuring instrument of the type mentioned above which reduces the time expenditure for the identification and determination of the absolute values of the individual reference marks. It is a further object of the present invention to provide a measuring instrument wherein the number of reference marks per measuring length can be efficiently increased.